In a traveling wave optical modulator, when light passes through the arm, modulation-signal superimposed microwaves also pass through the arm at the same velocity. At this time, the electric field of the microwaves is applied to the arm, and the light is modulated. Comparing with lumped-constant optical modulators, the traveling wave optical modulator can spread modulation bandwidth without the limitation by the capacity of the waveguide.
As described, in the traveling wave optical modulator, the waveguide is used as the microwave-transmitting line. For the propagation of a basic mode, the optimal thickness of the core layer and the width of the waveguide are about 0.2 μm and about 2 μm, respectively. In this case, however, the impedance of the waveguide is as low as 35Ω, causing mismatch of impedance with the impedance of the power supply line (normally 50Ω). As a result, the light inputted in the optical modulator is reflected or attenuated, and the modulation bandwidth is narrowed.
In order to elevate the impedance for microwave, it is required to thicken the core layer and to narrow the width of the waveguide. However, if the core layer is thick, the propagation mode of the light is not the basic mode, but is the higher mode. Then, the extinction ratio is deteriorated and the operating voltage is elevated so as not to operate as an optical modulator. Furthermore, if the width of the high-mesa waveguide is narrow, the light is not propagated, and loss in enlarged. As described above, although one waveguide is used for both light and microwave, the optimal sizes for both are different.
Also in a generally used traveling wave Mach-Zehnder optical modulator composed of lithium niobate (LiNbO3), since the permittivity of the material is low, the impedance of the waveguide can be 50Ω. Further in the lithium niobate modulator, in order to widen the modulation bandwidth, it is proposed for example that the termination resistance is lowered to lower the output impedance, or a stub is connected to the termination resistor (for example, refer to Japanese Patent Laid-Open No. 2004-170931, Japanese Patent Laid-Open No. 2007-010942, WO 2005/096077, Japanese Patent Laid-Open No. 11-183858, Japanese Patent Laid-Open No. 07-221509, and WO 2010/001986).